This invention relates in general to an improved process for the production of acetic acid or a derivative thereof by liquid phase oxidation of methane. In particular the present invention relates to the liquid phase oxidation of methane with an oxidant in a strong acid in the presence of a catalyst comprising palladium combined with a promoter.
The primary process route used today for production of acetic acid is by catalytic reaction of methanol and carbon monoxide. Such a process, typically termed “carbonylation”, is described in a number of patents and publications. Rhodium, palladium or iridium-containing catalysts have been found especially useful for conducting this reaction. A recent example of a patent on such a process is U.S. Pat. No. 6,472,558 of Key et al., which describes a process for reaction of methanol (and/or a reactive derivative of methanol such as methyl acetate or dimethyl ether) and carbon monoxide in a liquid reaction composition that comprises methyl acetate, methyl iodide, acetic acid, water and a polydentate phosphine oxide, in addition to the iridium catalyst.
Another process route that has been found useful for the production of acetic acid involves the catalytic oxidation of ethane and/or ethylene. Such processes are disclosed, for instance, in U.S. Pat. No. 6,383,977 of Karim et al., U.S. Pat. No. 6,399,816 of Borchert et al. and U.S. Pat. No. 6,706,919 of Obana et al. and U.S. published applications 2003/0158440 of Zeyss et al. and 2004/0031084 of Cook et al. In the processes described in these, a mixed oxide catalyst containing multiple metals such as palladium, molybdenum, vanadium, niobium, antimony, nickel, calcium, and others, is used.
Methane is the lowest molecular weight, and simplest in structure, of the hydrocarbons. Because of the existence of large reserves of methane worldwide it has been considered desirable for some time to develop processes to convert methane to more valuable chemicals. Processes for production of acetic acid from methanol represent an ultimate use of methane, but in current commercial practice, the methane first must be converted to methanol. A process that produces acetic acid directly from methane would be more desirable.
A small amount of work has been conducted so far on the direct conversion of methane to acetic acid, for instance by reaction of methane with carbon dioxide. A process for production of acetic acid by such a reaction was disclosed in the 1924 British patent 226,248 of Dreyfus. The patent describes a process involving gas phase reaction of methane with carbon monoxide and/or carbon dioxide in the presence of a catalyst that preferably contains nickel carbonate. Apparently a mixture of acetic acid, acetaldehyde and possibly acetone is obtained. No data on yields or conversions is contained in this patent.
PCT application WO 96/05163 of Hoechst A. G. describes a gas phase reaction of methane and carbon dioxide to produce acetic acid, using a catalyst containing one or more Group VIA, VIIA and/or VIIIA metals. Selectivities of 70–95% based on methane are asserted; however the application contains no exemplary data.
A number of researchers have investigated production of acetic acid by liquid phase carbonylation of methane with carbon monoxide, due to the favorable thermodynamics of this reaction. See, for instance, Bagno, et al. J. Org. Chem. 1990, 55, 4284–4289; Lin, et al., Nature 1994, 368, 613–615, Chaepaikin, et al., J. Mol. Catal. A: Chem. 2001, 169, 89–98; Nishiguchi, et al., Chem. Lett. 1992, 1141–1142; Nakata, et al. J. Organomet. Chem. 1994, 473, 329–334; Kurioka, et al., Chem. Lett. 1995, 244; Fujiwara, et al., Studies in Surface Science and Catalysis 1998, 119, 349–353; Taniguchi, et al., Org. Lett. 1999, 1 (4), 557–559; Asadullah, et al., Tetrahedron Lett. 1999, 40, 8867–8871; Nizova et al., Chem. Commun. 1998: 1885; Piao et al., J. Organomet. Chem. 1999, 574, 116–120; Yin et al., Appl. Organomet. Chem. 2000, 14, 438–442; Reis et al., Angew. Chem. Int. Ed. 2003, 42, 821; and Asadullah, et al., Chem. Int. Ed. 2000, 39 (14), 2475–2478.
Recently, Periana et al., Science, 2003, 301, 814 reported an experiment in which acetic acid was directly prepared from methane in the presence of palladium sulfate in concentrated sulfuric acid without the addition of COX. Acetic acid and methyl bisulfate were the reaction products. Hydrolysis of these yields acetic acid and an equilibrium mixture of methyl bisulfate and methanol. After 7 h of reaction at 180° C., 82 mM acetic acid and 38 mM methanol were ultimately formed. The reaction as reported was 90% selective to these products, with the only byproduct being CO2. The overall stoichiometry of the reaction is given as:2CH4+4H2SO4→CH3COOH+4SO2+6H2O.
However, this process displayed a serious drawback. During the reaction particles of palladium black were formed due to the reduction of Pd(II) to Pd(0). This results in loss of catalytic activity due to loss of soluble palladium. A further drawback to the process is that it consumes sulfuric acid and produces SO2 as a by-product, which is either wasted or can be recycled back to sulfuric acid via a number of processing steps including reaction with oxygen to give SO3 which is then reacted with water to give sulfuric acid. It would be desirable to provide a process for production of acetic acid from methane using a palladium catalyst where the catalytic benefits of palladium salts are retained but without the disadvantages of palladium black formation. It would also be desirable to reduce the number of processing steps.